1. Field of the invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for cell reselection in a wireless communication system.
2. Related Art
3rd generation partnership project (3GPP) long term evolution (LTE) is an improved version of a universal mobile telecommunication system (UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) that is an evolution of the 3GPP LTE.
A closed subscriber group (CSG) is introduced to provide better quality of service by allowing limited access only to a specific subscriber. A base station capable of providing a CSG service is referred to as a home eNodeB (HNB), and a cell providing a licensed service to subscribers of the CSG is referred to as a CSG cell. Basic requirements of the CSG in 3GPP are disclosed in 3GPP TS 22.220 V1.0.1 (2008-12) “Service requirements for Home NodeBs and Home eNodeBs (Release 9)”.
Cell selection is a process of selecting a cell in which a service is provided to a user equipment. Cell reselection is a process of reselecting a cell in a state where the user equipment has already selected the cell by completely performing the cell selection. Hereinafter, the cell selection and the cell reselection have the same meaning in terms of selecting a new cell unless specified otherwise.
The CSG cell has a smaller cell size (i.e., a size of coverage provided by the cell) than a normal cell. As cells with various cell sizes coexist, the cell reselection may be ineffectively performed when the conventional cell selection is performed.